1. Field of the Invention
This invention relates to a liquid crystal composition and a liquid crystal display element using the same. More particularly, it relates to a nematic liquid crystal composition for liquid crystal display elements according to active matrix mode needing a high reliability, and a liquid crystal display element using the liquid crystal composition.
2. Description of the Related An
As the development theme directed to LCD (liquid crystal display device), two points of a high density display (high contrast) and a high speed response have been mentioned, and have been researched along with the display mode. Among LCDs, active matrix liquid crystal display (AM-LCD) such as TFT (thin film transistor), etc., has been advanced in the aspects of colorization and higher densification and has been regarded as a favorite of flat panel display. However, even this AM-LCD has been far inferior in the aspects of densification, response speed, size of display area, etc., to CRT having been most prevailing now. Thus, active researches have been carried out into various elements constituting AM-LCD such as driving circuit, switching element, color filter, etc. Further, for liquid crystal materials, various specific features which cannot be satisfied by conventional materials such as biphenyls, PCHs, etc., having cyano group have been required.
The present inventors have considered that, in the case of compounds having cyano group(s) as terminal group or side chain, this cyano group has a certain interaction with ionic impurities present in display elements, and as a result, has a bad influence upon current value and specific resistance value and hence display contrast. Thus, compounds having a cyano group cause reduction in the reliability of elements, concretely increase in the current consumption and reduction in the specific resistance value in cooperation with the driving current of two-terminal or three-terminal switching element, and also cause display unevenness and reduction in the contrast as regards the display specific features of the elements.
For example, the contrast in the TFT display element shown in FIG. 1 is closely related to its signal voltage-holding characteristics. The signal voltage-holding characteristics refer to the extent of reduction in the signal voltage impressed on the TFT pixels containing liquid crystals in a definite frame period.
Thus, when the signal voltage is not reduced, no reduction in the contrast occurs. Further, as to the signal voltage-holding characteristics, the less the storage capacity (Cs) provided in parallel to liquid crystal (LC) and the specific resistance of LC, synergistically the worse the voltage-holding characteristics. In particular, when the specific resistance of liquid crystal is the lower limit value or lower, the voltage-holding characteristics are exponentially made inferior to thereby extremely reduce the contrast. In particular, in the case where no storage capacity is added for the reason of simplification of TFT preparation process or the like, contribution of the storage capacity cannot be expected so that a liquid crystal composition having a particularly high specific resistance is required.
From such a viewpoint, a liquid crystal composition for AM-LCD composed only of compounds containing no cyano group is disclosed in, for example, Japanese patent application laid-open No. Hei 2-289682. Further, in Japanese patent application laid-open No. Sho 63-61083, particularly a number of compositions composed only of compounds having no cyano group among compounds included in a TN composition have been used.
However, LCD using such compositions has a high threshold voltage and hence is unsuitable to low voltage drive; thus a 5 V single drive is difficult and the LCD is insufficient for a display of a portable device driven by a battery. Further, since the composition has a high viscosity and the response speed of the LCD is slow, such drawbacks are raised that the display grade is reduced in the case of moving image display and it is impossible to respond to quick mouse movement or scrolling document in a word processor display.
As described above, in the case of AM-LCD using a liquid crystal composition composed only of compounds having no cyano group, it is difficult to attain a low threshold voltage sufficient for carrying out battery drive and a quick response speed sufficient for obtaining a high grade moving image display in office automation equipment, while satisfying the requirements of a small current consumption and a high specific resistance for retaining the reliability and the high contrast characteristic of the LCD.
An object of the present invention is to provide a liquid crystal composition having a high specific resistance and a small current consumption and yet a low viscosity.
Another object of the present invention is to provide a liquid crystal display element which uses the above liquid crystal composition and exhibits a high contrast, a high reliability, a quick response time and a high grade, moving image display and can correspond to quick movement of a mouse and scrolling in office automation equipment.